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Respiratory rates on the U. S. southeastern continental shelf have been estimated several times by different investigators, most recently by Jiang et al. (Biogeochemistry 98:101–113, 2010) who report lower mean rates thanwere found in earlier work and attribute the differences to analytical error in all methods used in earlier studies. The differences are, instead, attributable to the differences in the geographical scope of the studies. The lower estimates of regional organic carbon flux of Jiang et al. (Biogeochemistry 98:101–113, 2010) are a consequence of their extrapolation of data from a small portion of the shelf to the entire South Atlantic Bight. This comment examines the methodologies used as well as the variability of respiratory rates in this region over space and time

In August 1993, we measured photosynthesis, chlorophyll a, bacterial secondary production, microbial community respiratory rate, bacterial abundance, dissolved free amino acids, nitrate, phosphate, silicate, and dissolved oxygen in the eastern Chukchi Sea. Our cruise track was mostly in loose pack ice exceeding 50% ice cover, with heavier ice cover near 75° N. We sampled over the continental shelf and slope, in deep water in the Canadian Basin, and over the Chukchi Cap. Primary production was highest over the upper continental slope, averaging 748 mg C m-2 d-1. In deep water and heavier ice cover in the Canadian Basin, primary productivity averaged 123 mg C m-2 d-1. However, microbial community respiratory rates averaged 840 mg C m-2 d-1 over the upper slope and 860 mg C m-2 d- 1 in the Canadian Basin. Nitrate was virtually depleted in the upper mixed layer suggesting some nutrient limitation and dependence on regenerated ammonium in late summer. This is supported by f-ratios ranging from 0.05 to 0.38. Estimates of annual primary production of organic carbon, both from our 14C and 13C assimilation measurements and from the supersaturation of dissolved oxygen in the upper mixed layer at all stations suggest that significant primary production occurs well beyond the continental shelves out into the so-called perennial pack ice. Respiratory activity in the upper mixed layer exceeded primary productivity at the deep-water stations, as it often does in summer oligotrophic conditions at lower latitudes. These observations suggest that rates of both autotrophic and heterotrophic biological activity in the upper mixed layer of the deep waters of the Arctic Ocean may be considerably higher than suspected and should be incorporated into models of polar processes

We compared microbial community respiration and related parameters in the Gulf of Mexico in January and June 1993. Microbial community respiratory rates in the upper mixed layer varied from lt 0.03 mu-M O-2 h-1 in the central Gulf in January to 1.4 mu-M O-2 h-1 in the Mississippi River plume in June. Although higher respiratory rates were found in June than in January, no significant differences were found in bacterial numbers or mean cell volume. Dissolved free amino acid concentrations were an order of magnitude higher in June, but there was little difference in concentrations of phosphate or monosaccharides between January and June, Enrichment experiments in June showed phosphate to be the primary limiting factor for bacterial production and microbial community respiration and organic carbon substrates to be a secondary limiting factor. Respiratory rate and bacterial secondary production increased when phosphate was added to water samples. Ammonium, iron and other trace metals, vitamins and chelators had no effect. Glucose was utilized only when supplemented with phosphate. Turnover time of bacterial biomass in June, based on counts, sizes, and production data, was 7 to 30 h, with the shortest times at oligotrophic stations. The observed rates of bacterial respiration and production imply the utilization of multiple sources of organic and recycled inorganic nutrients in a complex and inefficient food web